The Mass of the Galaxy Can be determined using Kepler’s 3 rd Law –Solar System: the orbital velocities of planets determined by mass of Sun –Galaxy: orbital velocities of stars are determined by total mass of the galaxy contained within that star’s orbit Two key results: –large mass contained in a very small volume at center of our Galaxy –Much of the mass of the Galaxy is not observed consists neither of stars, nor of gas or dust extends far beyond visible part of our galaxy (“dark halo”)

Galaxy Masses Rotation curves of spiral galaxies comparable to milky way Masses vary greatly

The Missing Mass Problem Dark Matter is dark at all wavelengths, not just visible light The Universe as a whole consists of up to 25% of Dark Matter!  Strange! What is it? –Brown dwarfs? –Black dwarfs? –Black holes? –Neutrinos? –Other exotic subatomic particles? Actually: Most of the universe (70%) consists of Dark Energy  Even stranger!

Missing Mass Problem Keplerian Motion: more distance from center  less gravitational pull  slower rotational speed Actual data Hypothetical Keplerian motion

The Tully-Fisher Relation A relation between the rotation speed of a spiral galaxy and its luminosity The more mass a galaxy has  the brighter it is  the faster it rotates  the wider the spectral lines are Measuring rotation speed allows us to estimate luminosity; comparing to observed (apparent) brightness then tells us the distance

Beyond the Galactic Scale – Clusters of Galaxies The Local GroupThe Virgo Cluster

Superclusters

Beyond Superclusters Strings, filaments, voids Reflect structure of the universe close to the Big Bang Largest known structure: the Great Wall (70 Mpc  200 Mpc!)

Cosmology The part of astronomy (and astrophysics) that deals with the greatest structures in the universe – and the evolution of the universe itself!

Cosmologically relevant Questions –What is in the universe? –How do these things interact? –How does the universe change in time? Is there a beginning? Is there an end?

What’s in the Universe? THE UNIVERSE clusters and superclusters voids galaxies like the Milky Way quasars Stars nebulae molecular clouds star clusters Solar System black holes pulsars Sun planets moons comets meteors asteroids dust terrestrial jovian Big ……………………………………..small

So, why is the night sky dark? (Olbers’ Paradox) Conclusion: either –Universe is not infinite or –Universe changes in time

Observation III: Everything is moving away from us! Measure spectrum of galaxies and compare to laboratory measurement lines are shifted towards red This is the Doppler effect: Red-shifted objects are moving away from us

Hubble’s Law The final rung on the cosmic distance ladder Hubble’s observations (1920’s): –Light from distant galaxies is red- shifted –The more distant the galaxy, the greater the red-shift Interpretation: –Galaxies are moving away from us –More distant galaxies are moving faster The universe is expanding, carrying the galaxies with it!

Hubble, 1929 Doppler Shifts of Galaxies

Hubble’s Law H 0 = (68 ± 1) km/sec/Mpc is Hubble’s constant Compare to distance = velocity  time Appears the universe “exploded” from a single point in the past – the Big Bang Age of the universe is 1/H 0 or about 14 billion years Velocity = H 0  Distance Distance = Velocity /H 0

Example: Object that is 2 Mpc away recesses with v = H d = (68 km/s/Mpc) (2Mpc) = 136 km/s Object that is receding with 27,200 km/s has distance d = v/H = (27,200 km/s)/ (68 km/s/Mpc) = 400 Mpc